This invention relates to frame structures for construction vehicles in general, and more particularly it deals with a frame structure for construction vehicles suitable for use in construction or civil engineering work which require adjustments of the width of the vehicles to be effected.
A construction vehicle suitable for use in construction work equipped with a pair of endless tracks each track disposed on one of opposite sides of the vehicle body and having a bucket or other operating tool mounted thereon, such as an excavator, is generally referred to as a crawler vehicle. In order that tumbling of the vehicle body may be avoided during operation, such crawler vehicle must be able to adjust the distance between the two endless tracks (hereinafter referred to as a vehicle body width) in such a manner that the vehicle body width can be increased when the vehicle is in an operation mode, while the vehicle body width can be reduced to a level within the limits suitable for transportation according to government regulations when the vehicle is in a transportation mode for transportation to a faraway destination.
To this end, various frame structures have been proposed which are provided with vehicle body width adjusting devices. For example, Japanese Utility Model Application Laid-Open Number 156035/77 discloses a frame structure including a vehicle body width adjusting device. However, such a frame structure suffers the following disadvantages:
(1) The frame structure comprises elongated box-shaped members mounted on a lower frame, and support legs mounted on a pair of track frames disposed on opposite sides of a vehicle body and each supporting one of the elongated box-shaped members while being inserted in the elongated box-shaped member. With this frame structure, it is necessary to machine with precise finishes four surfaces or upper and lower surfaces and left and right surfaces of each of the support legs (outer peripheral surfaces) and each of the elongated box-shaped members (inner peripheral surfaces). This machining operation is time consuming and involves great expenses. Particularly, machining of the inner peripheral surfaces of the elongated box-shaped members is troublesome and time consuming, and difficulties are experienced in increasing the precision with which machining is effected.
(2) The elongated box-shaped members each have the support legs inserted in opposite ends thereof. Thus the opposite ends of each elongated box-shaped member should be open. This construction is very low in strength with respect to torsion loads, so that it becomes necessary to increase the size of the cross-sectional area or increase the thickness of plates of the member to increase strength. Moreover, the elongated box-shaped member and the support legs overlap one another in part of them, thereby increasing weight. Thus an incease in weight caused by the need to increase strength and an increase in weight due to the presence of overlaps combine to cause a large increase in the weight of the frame structure, thereby increasing cost.
(3) The bending moment applied to the lower frame and the track frames connected to one another during operation of a vehicle will be discussed. Assuming that a load F is equally divided into F/2 and F/2 which are each borne by one of the left and right track frames, the bending moment would be maximized near the central portion of the frame and minimized at opposite ends thereof. The elongated box-shaped member is connected by bolts at opposite ends thereof to the support legs in a position near the portion of the frame in which the bending moment is maximized, so that the need arises to increase the strength of the connections.
(4) As stated in paragraph (1), difficulties are encountered in increasing the precision with which the elongated box-shaped members are finished, and this makes it necessary to provide a large clearance between sliding portions of the elongated box-shaped member and the associated support legs. In this case, they are clamped together by bolts to prevent relative movements therebetween. However, loads would be applied to the bolts by the weight of the vehicle body, reactions produced during operation (such as reactions of excavation operation) and impacts applied during travel, so that shearing, bending, tensile and other forces would act on the bolts. As a result, the bolts would tend to be loosened or broken. When the bolts are loosened or broken, relative movements would occur between the elongated box-shaped member and the support legs during operation or travel, so that the performance, stability and riding comfort of the vehicle would be adversely affected.
(5) As stated in paragraph (4), a clearance should be provided between the sliding portions of the elongated box-shaped member and the support legs and they should be clamped together by bolts to avoid relative movements therebetween. In this case, the bolts used should be increased in number and the clamping torque of the bolts should also be increased greatly because the relative movements caused by a horizontal load in particular should be absorbed by friction between the elongated box-shaped member and the support legs. Thus clamping should be effected by using a wrench of large size and heavy weight or other similar tool. Being large and heavy, such wrench is difficult to operate. Als it often happens that the clamping operation should be performed in places of inconvenience for operation. This increases the time required for effecting vehicle body weidth adjustments to about 5 hours in the case of a crawler vehicle of a bucket capacity of 2 m.sup.3 class, for example.